1. Field of the Invention
This invention relates generally to systems and methods for detecting irregularities on a substrate. More particularly, this invention relates to systems and methods for detecting irregularities on the surface of silicon wafers or photomasks.
2. Description of the Related Art
Semiconductor wafers are inspected prior to, during, and after patterning procedures. Optical inspection systems typically employ illumination optics and collection-detection optics for directing incident light from a light source onto a wafer to be inspected, and observing returned light.
Imaging techniques are often employed in modern inspection systems. These systems are classified according to the direction of the illumination with respect to the collection optics. If the illumination impinges upon the substrate from a direction such that the transmitted or specularly reflected light is collected by the imaging optics, the system is termed “bright field” (BF). If, on the other hand, the transmitted or specularly reflected light arrives from a direction, which is outside the collection angle of the collection optics, the system is termed “dark field” (DF).
Today, semiconductor wafers are inspected using bright-field techniques, dark-field techniques, or combinations thereof Coherent light is commonly employed for illumination.
Lasers, which are commonly used in inspection systems produce undesirable coherent phenomena in imaging systems, such as ringing of edges and speckles. Schemes exist for destroying the coherence of laser sources, but they inevitably add to the system's complexity and reduce optical power. Laser-scanning inspection systems typically employ a focused laser spot scanning over the sample. The reflected or scattered light is collected by a detector, which may be non-imaging, e.g., a photomultiplier tube, partially imaging, e.g., a linear CCD, or fully imaging, e.g., an area CCD. Each of these entails certain advantages and limitations.
With a non-imaging detector, the system resolution is determined solely by the illuminated area, as all the collected light is integrated into a single signal. This scheme precludes multi-spot, line and area illumination schemes. Throughput is limited by the spot size and the scan rate. Usually the beam is scanned over the sample using a rotating polygon mirror, acousto-optic device scanner, or oscillating galvanometric scanning mirror.
With a partially imaging or fully imaging detector, light is collected simultaneously from a larger region of the sample so that multiple spots may be illuminated simultaneously, multiplying the previous throughput by the number of illumination spots. However, if these spots are not spatially separated, image distortions may occur due to coherent interference effects.
It may be advantageous to simultaneously collect both the reflected or bright-field image, at high spatial resolution at high throughput, and the scattered or dark-field image, at lower spatial resolution at low throughput. The advantage is related to the difference in throughput achievable with optimal detectors for the bright-field image (partially imaging or fully imaging), as compared with the dark-field (non-imaging). One method for achieving simultaneous bright- and dark-field detection is addressed in commonly assigned U.S. Pat. No. 6,122,046 to Almogy, the disclosure of which is herein incorporated by reference, wherein a large spot on a substrate is illuminated using optics having a low numerical aperture (NA). Dark-field detection is achieved by a non-imaging detector. The dark-field resolution is determined by the illumination spot size. The bright-field signal is collected with high NA optics, and imaged onto an area detector, providing improved resolution relative to the dark field detector by a factor that is the ratio of the collection NA to the illumination NA. However, when using imagine detectors, the bright-field images may suffer undesirable distortion due to coherence effects in the form of ringing at feature edges, if there is mismatch between the NA of the collection optics and the imaging optics. When detection is performed with non-imaging detectors, which generally have lower resolution than imaging detectors, these coherence effects can usually be disregarded.